In the wireless communication system operating in Transmission Time Interval (TTI) mode, the transmitter transmits a message at TTI, and the receiver performs Cyclic Redundancy Check (CRC) on the message received from the transmitter and reports the result to the transmitter. That is, the receiver in acknowledge mode determines whether the message has been received successfully and transmits an acknowledgement/negative acknowledgement (ACK/NACK) message to the transmitter based on the determination result.
In Long Term Evolution (LTE) system, Radio Resource Control (RRC) is responsible for controlling layer 3 signaling between User Equipment (UE) and evolved Node B (eNB), connection establishment and release, broadcast of system information, Radio Bearer establishment/reconfiguration and release, RRC connection mobility procedure, and the like. Typically, the RRC messages are exchanged in TTI mode, and the transmitter performs retransmission process depending on the response transmitted by the receiver.
A description is made of RRC message transmission procedure. FIG. 1 illustrates a sequence diagram of the operations of a UE and network entities for a handover procedure in an LTE system.
Referring to FIG. 1, a UE is communicating packet data 111 with a source eNB. During the communication, the source eNB sends a Reference Signal Received Power (RSRP) Detection signal 112 to the UE. Upon receipt of the RSRP Detection signal, the UE performs a handover measurement procedure 121 to 129 with the source eNB. During the handover measurement procedure, the UE performs Scheduling Request, Buffer Status Report, and Measurement Report to the source eNB. The source eNB sends an ACK/NACK message to the source eNB in response to the individual messages.
If a handover decision has been made, the source eNB performs a handover preparation procedure 131 to 133 with a target eNB. In the handover procedure, the source eNB sends a Handover Request message 131 to the target eNB, and the target eNB sends a Handover Request ACK message to the source message.
Once the handover has been prepared, the source and target eNBs and UE perform a handover interruption procedure 141 to 150. During the handover interruption procedure, the source eNB sends an RRC connection reconfiguration message to the UE and transfers a PDCP SN status message to the target eNB. The source eNB also forwards the data to the target eNB. After the establishment of a link between the UE and target eNB, the target eNB and the UE perform packet data communication. Thereafter, the target eNB sends a packet switch request message to a Mobility Management Entity/Source Gateway (MME/SGW) and performs then performs packet data communication with the UE.
In FIG. 1, the messages 123, 126, 141, and 148 are RRC messages transmitted in TTI mode, and the receiver sends an ACK/NACK message in response to the individual RRC messages. When a NACK message is received, the transmitter retransmits the corresponding message.
FIG. 2 illustrates a message flow diagram for an exchange of RRC messages and ACK/NACK messages between a UE and an eNB in a conventional wireless communication system operating in TTI mode. In FIG. 2, the transmission party receives an ACK/NACK message in 8 TTIs after the transmission of an RRC message.
Referring to FIG. 2, the eNB sends an RRC message 221 to the UE. If the RRC message has been received, the UE checks the normality of received RRC message and sends an ACK/NACK message 213 in response to the RRC message depending on the normality check result. The eNB receives the ACK/NACK message in 8 TTIs after the transmission of the RRC. If the received RRC message is abnormal, the UE sends the NACK message to the eNB in response to the RRC message. Upon receipt of the NACK message, the eNB, as the transmitter, retransmits the RRC message 215 and receives the ACK/NACK message 217 in another 8 TTIs after the retransmission of the RRC message.
Since the eNB and UE communicate with each other through a radio link, the message transmitted by the eNB or UE can be lost due to the radio link problem or bad communication condition. For example, if the RRC message transmitted at step 211 has been erroneous or lost, the eNB must retransmit the RRC message at step 215, resulting in procedure delay of 8 TTIs. This means that the message loss causes the retransmission procedure delay equal to the time interval for retransmission in the periodical retransmission mode.
Assuming that the RRC message of FIG. 2 is the RRC connection reconfiguration message of FIG. 1 as a handover command and if the RRC connection reconfiguration message transmitted by the source eNB has been lost, the UE sends the NACK message to request the source eNB for the retransmission of the lost RRC message. Once the NACK message has been received, the source eNB retransmits the corresponding RRC message at a predetermined time, i.e., 8 TTIs after the initial transmission of the RRC message, resulting in handover interruption time.
There is therefore a need for a method to reduce the handover interruption time by minimizing the retransmission times of RRC message.